The present invention relates to a tail packing for the shield construction method, and more particularly to an improvement of the tail packing for the slurry pressurized shield method.
Recently in urban communities, due to necessity of securing surface road transportation, prevention of pulbic nuisance such as from vibration and noise, crossings of existing structures and difficulty of construction posed by close proximity of other works, construction conditions have changed drastically, accelerating development of the shield method replacing the conventional open-cut method.
The shield method was orginally contrived and developed as a highly technical tunnelling method to cope with the worst type of ground such as under rivers and ocean bed.
The shield construction is a tunnel boring method in which a steel tubular frame is used to sustain the earth pressure while the earth is excavated. The tunnel is constructed by advancing the shield as construction progresses. Namely, in the shield construction, the earth is excavated for the length equivalent to the width of one ring or piece of segment, which is a concrete block used as tunnel lining and serves to counter-balance reaction by shield jacks, and the shield machine is advanced for a distance equivalent to a single ring of the segment. Then, the segments are assembled at the tail end of the shield machine, and backfill grouting operations are then carried out, in which the space between segments and the natural ground is grouted with mortar or concrete. After the backfill grouting is carried out, hauling out of dross (namely, excavated soil) and hauling in of additional segments, etc. are performed.
The shield method has such advantages that the surface above is not affected during construction since most of the work is performed underground, that construction can be accomplished even where buried utility lines such as underground cables are numerous since the construction deep under the ground is possible, and that construction nuisance such as noise and vibration is less compared to the other tunnel construction method.
However, in recent times, in view of safety and sureness of natural ground stabilization, prevention of settlement, speed-up of construction and reduction of labor, efforts have been made to achieve a new construction technique, resulting in the development of the slurry shield or slurry pressurized shield, which will be described.
Particularly, it has been found that the conventional shield method has faced with the serious problem to be solved of prevention of settlement.
One of the most serious problems inherent to the shield method is settlement or land subsidence due to the collapse at the tail void of the shield machine. This settlement is induced by a collapse of the earth to tail void formed between the segment and the earth after the shield machine is advanced.
In order to prevent the collapse of the earth at the tail void, backfill material such as mortar is grouted. However, a tail packing of the shield, which is not only important for preventing a reverse flow of the backfill and permitting an instant backfill grouting but also is necessary for prevention of slurry leakage in case of a slurry pressurized shield method, did not have a desired shield performance or sealing effect. Lack of desired shield performance in the tail packing causes a hindrance in the instant backfill grouting operation, resulting in the aforementioned serious problem of settlement. In particular, the conventional tailpacking was not effective for sealing when deformed segments are used or when there was an unevenness between the segments. Thus, three involved a serious problem that instant filling of backfill material was hampered.
Attempts have been made so as to improve the conventional tail packing to permit a prevention of leakage of the backfill material into the shielded area.
A brush-type tail seal packing was developed to meet with the above requirement, as described in Japanese Utility Model Application No. 49-58618, laid open for public inspection on Dec. 6, 1975 under No. 50-147431. The brush-type tail packing is composed of a number of nylon monofilaments or the like which are arranged in close proximity and dense manner to form a "brush" shape. It has been found that the brush-type tail seal packing permits a desirable shield effect in the aforementioned shield construction method. However, the conventional brush-type tail seal packing is not available in the slurry pressurized shield method.
The slurry pressurized shield is a tunnelling method characterized by the use of slurry for the stabilization of the area to be cut by the excavator and which simultaneously enables transport of dross as a fluid. The synopsis of this method is as follows: a separator wall is installed at the rear of the rotary cutter fixed on the nose of the shield machine; the space between the separator wall and the cutter face is filled with bentonite particles and the earth retaining effect is achieved by formation of a mud-cake caused by activity of bentonite; the earth-pressure and the ground water pressure at the cutting area is restrained by the pressurized slurry and the cutter-face; underwater excavation is performed by the rotary cutter and the dross piped out together with the slurry.
By this method, as the slurry is constantly circulated to perform the excavation, a control pump and pipeline is necessary to maintain the pre-determined slurry pressure at the cutting area.
In the slurry pressurized shield method, since the stabilization of the facing, namely prevention of collapse of the facing, is effected by the slurry pressure, the slurry leakage causes a lowering of the slurry pressure, and results in a collapse of the facing. The pressurized slurry is fed along the outer surface of the shield to the rear side of the shield, and therefore, it is necessary to completely seal the slurry and to prevent the slurry from leaking out of the tail packing.
This is the reason why the aforementioned conventional brush-type tail packing is not available for prevention of the slurry leakage in the slurry pressurized shield method, though the same has an effect to seal the backfill material as mortar.
In addition to the above, the conventional brush-type tail packing is lacking in the necessary stiffness, durability, and endurance against pressure.